A Huffington Post writer asked this question in a headline Wednesday: "If Trump is winning, why aren't congressional Republicans losing primaries?" His post sought to answer the question and drew upon the expertise of Robert Boatright, associate professor of political science, to help with that.

Here, an excerpt:

"But isn’t it surprising if no true GOP challenger beats an incumbent this primary season, when roughly 20 million of 30 million Republican primary voters went for outsiders Trump and Sen. Ted Cruz (R-Texas)?

"Not really, political scientists say.

"Robert Boatright, the author of Getting Primaried: The Changing Politics of Congressional Primary Challenges, told HuffPost that primary challengers are most often driven by outside groups. 'I think the sorts of groups that have financed primary challenges in previous years are probably preoccupied with aiding Republican incumbents in states where Trump may be a drag on the ticket,' Boatright said.

“ 'Trump basically has so dominated people’s attention this year that conservative groups haven’t really tried to mount any challenges,' he added."